The present invention relates to a projection exposure apparatus and, more particularly, to a projection exposure apparatus that uses far ultraviolet light for pattern formation in the process of producing semiconductor devices (IC, CCD, etc.), liquid crystal display devices, thin-film magnetic heads, and so forth.
The demand for larger-scale integration of semiconductor devices has been increasing year by year, and the pattern rule (i.e., a line width of a pattern) of the required circuit patterns has been correspondingly decreasing. It is known that the line width that can be resolved by a projection optical system reduces in proportion to the wavelength. Therefore, in order to form a circuit pattern of smaller pattern rule by photo-lithography process, it is only necessary to shorten the wavelength of light used for exposure. At the present, an exposure apparatus in which a KrF excimer laser having a wavelength of 248 nm is used as a light source has already been developed. Further, a mercury lamp having a wavelength of about 220 nm or 184 nm, an ArF excimer laser having a wave length of 193 nm and the like have been noted as a short wavelength light source.
In conventional exposure apparatuses in which g-ray (having a wavelength of 436 nm), i-ray (having a wavelength of 365 nm), a KrF excimer laser or a mercury lamp emitting light having a wave length of about 250 nm is used as a light source, since the wavelengths of these light beams are not overlapped with an absorption spectrum zone of oxygen, there is no inconvenience such as reduction in light available rate caused when light is absorbed by oxygen molecules in a light path and/or generation of ozone due to light absorption of the oxygen molecules.
However, in the light source such as the ArF excimer laser, since light emitting spectrum is overlapped with the absorption spectrum zone of oxygen, the above-mentioned reduction in light available rate and/or generation of ozone due to light absorption of the oxygen molecules will occur. For example, if it is assumed that transmittance of the ArF excimer laser beam in the vacuum or in inert gas such as nitrogen or helium is 100%/m, in a free-run condition (natural light emitting condition). i.e., in an ArF wide range laser, the transmittance becomes about 90%/m, and, even when an ArF narrow band laser is used for reducing a spectrum width to avoid absorbing lines of oxygen, the transmittance is decreased to about 98%/m.
It is considered that the reduction in transmittance is caused by influences of absorption of light caused by the oxygen molecules as well as generation of ozone. The generation of ozone not only affects a bad influence upon the transmittance (light available rate) but also worsens performance of the apparatus due to reaction to a surface of optical material or other components of parts.
In such exposure apparatuses, in order to prevent the reduction in transmittance and/or generation of ozone by reducing oxygen density in the light path, it is well known that a space including the entire light path must be filled with inert gas such as nitrogen (for example, refer to Japanese Patent Laid-open No. 6-260385 corresponding to U.S. Ser. No. 206,618 filed on Mar. 7, 1994).
FIG. 15 schematically shows a construction of an exposure apparatus (optical systems associated with illumination and image focusing are mainly illustrated and other parts are omitted from illustration). A light beam from an ArF excimer laser light source 201 is changed to a predetermined form by a beam shaping lens 202 and then is reflected by a mirror 203 to be incident on a beam expander lens 204. The light flux incident to the beam expander lens 204 is expanded or enlarged to a predetermined magnitude and then is reflected by a mirror 205 to be directed to a fly-eye lens 206 as an optical integrator, where illuminance is made uniform and an illuminating range is determined. Light from the fly-eye lens 206 is focused on a reticle conjugate surface by a first relay lens 207. The reticle conjugate surface is provided with a reticle blind 208 for regulating or limiting an exposure range. Light passed through the reticle blind 208 is illuminated onto a reticle 212 through a second relay lens 209, a mirror 210 and a main condenser lens 211. Light having passed through the reticle 212 is illuminated onto a wafer 214 through a projection lens 213, thereby focusing an image of the reticle 212 on the wafer 214.
FIG. 16 is a sectional view of an illumination optical system of the exposure apparatus, showing a light path from the ArF excimer laser light source 201 to the main condenser lens 211. A frame 221 contains optical parts such as the beam shaping lens 202 constituting the illumination optical system and is attached to the ArF excimer laser light source 201 via a bellows 223. Nitrogen gas from a nitrogen gas supply source 224 is supplied from one side of the frame 221 (i.e., a side to which the laser light source 201 is attached in FIG. 16) through a piping L201a and is discharged to a discharge device 225 from the other side of the frame 221.
In FIG. 16, while various optical parts were shown with simplification, actually, as shown in FIG. 17 (fully described later), each of the optical parts is constituted by a plurality of lenses which are integrally secured to the frame 221 by a support blocks 237. In FIG. 16, the reflection mirror 210 and the main condenser lens 211 are secured to the frame 221 by using a same support block 237h, and the other optical parts are secured to respective support blocks 237a-237g. 
Each of the optical parts secured to the frame 221 forms respective optical block at each of the support blocks 237a-237h, and maintenance (such as replacement) is effected for independent block. Lids 222a, 222b, 222c serves to close openings (through which the optical blocks are inserted and removed when the optical blocks are mounted and dismounted with respect to the frame 221) formed in the frame 221, so that the interior of the frame 221 is sealed by the lids 222a, 222b, 222c. Incidentally, although not shown, O-rings or packings are disposed between the frame 221 and the lids 222a, 222b, 222c to improve sealing ability.
FIG. 17 shows an example of the optical parts. Lenses 232a, 232b, 232c are successively inserted into a lens barrel 231 and are secured by a hold-down ring 234. Incidentally, there are provided separation rings 233a, 233b for maintaining predetermined distances between the lenses. Vent holes 235a, 235b, 236a, 236b formed in the lens barrel 231 and the separation rings 233a, 233b serve to introduce inert gas between the lenses. When the nitrogen gas is supplied into the frame 221, the nitrogen gas also flows into the lens barrel 231 through the vent holes 235a, 235b, 236a, 236b to replace the air between the lenses by the nitrogen gas. The lens barrel 231 is secured to the support block by set screws 238.
However, in the illumination optical system of the exposure apparatus shown in FIG. 16, even when maintenance regarding at least one of the parts disposed in the frame 221 is effected, the entire interior of the frame 221 is exposed to atmosphere. Thus, a large amount of nitrogen gas contained within the frame 221 escapes or leaks outside, with the result that it takes a long time to re-fill the nitrogen gas in the frame 221 after the maintenance. Further, it is very difficult to judge whether the frame 221 is filled with the nitrogen gas sufficient to not affect an influence upon the exposure.
To solve the problem, it is conceivable to increase the number of hermetic blocks to thereby reduce the volumetric capacity of each block. However, merely increasing the number of blocks causes an increase in the number of transparent windows defining the boundary between each pair of adjacent blocks. Further, each transparent window also has a predetermined transmittance. Therefore, as the number of windows increases, exposure light is increasingly attenuated by the windows, resulting in an increase in the loss of light intensity.
Further, in the conventional apparatus disclosed in the above-mentioned JP(A) 6-260385, no particular hermetically sealing device is provided for the space between the projection optical system and a substrate as a workpiece, but an inert gas is blown into the optical path of exposure light in the space, thereby carrying out gas replacement for the space.
A conventional projection exposure apparatus of the type described above is usually provided with a focus detecting system in which measuring light is incident obliquely on a substrate, and the reflected light from the substrate is received to measure the heightwise position of the substrate (disclosed in detail in JP(A) 60-168112; corresponding to U.S. Pat. No. 4,650,983). The conventional projection exposure apparatus is further provided with an alignment system in which alignment light is applied to a mark on the substrate, and diffracted or scattered light from the mark is received to measure the position of the substrate, and a laser interferometer in which laser light is applied to a moving mirror provided on a substrate stage, and the reflected light from the moving mirror is received to measure the position of the substrate stage (the alignment system and the laser interferometer are disclosed in detail in JP(A) 60-186845).
However, in such a conventional projection exposure apparatus, if an inert gas is blown into the space between the projection optical system and the substrate, fluctuation is induced in the atmosphere by variation of the gas flow velocity or other cause, which may result in an error in values measured by the above-described various measuring systems (focus detecting system, alignment system and interferometer) that use measuring light passing through the space between the projection optical system and the substrate or somewhere around it.
A first object of the present invention is to provide a projection exposure apparatus in which a hermetic space extending from a light source of an illumination optical system to the mask-side end of a projection optical system is divided into a plurality of hermetic blocks, each having an inert gas sealed therein, by using a plurality of partition devices with respective openings in place of a plurality of windows as used in the conventional apparatus, thereby enabling only a desired block to be opened, and thus making it possible to eliminate waste of a replacement gas and to prevent loss of exposure light intensity which would otherwise be caused by a plurality of windows.
A second object of the present invention is to provide a projection exposure apparatus which is arranged as described above and in which the space between the substrate-side end of the projection optical system and a substrate is defined as a hermetic space having an inert gas sealed therein by a predetermined hermetically sealing device, thereby making it possible to eliminate the influence of fluctuation which has heretofore been induced in the atmosphere near the optical path of measuring light used by various optical measuring sensors when an inert gas is blown into the space between the projection optical system and the substrate.
A third object of the present invention is to provide an exposure apparatus and an optical system for such an exposure apparatus, in which a time period required for replacing air by inert gas in a frame during the maintenance can be reduced and the replacing operation can easily be performed.
To attain the first object thereof, the present invention provides a first projection exposure apparatus including an illumination optical system for applying light of a specific wavelength to a mask formed with a pattern, and a projection optical system for projecting a pattern image of the illuminated mask onto a substrate. The projection exposure apparatus further includes a hermetically sealing device for shutting off from the atmosphere an optical path extending from a light source of the illumination optical system to the mask-side end of the projection optical system, and a partition device for partitioning the space in the hermetically sealing device to form hermetic blocks as occasion demands.
According to a preferred embodiment of the abovedescribed first apparatus, the partition device has a partition wall which is approximately perpendicular to the optical path, and a device for hermetically closing an opening provided in the partition wall at a position coincident with the optical path as occasion demands.
According to the first projection exposure apparatus of the present invention, the optical path extending from the light source in the illumination optical system to the mask-side end of the projection optical system is shut off from the atmosphere by the hermetically sealing device. Further, the space in the hermetically sealing device can be partitioned by a plurality of partition devices to form a plurality of hermetic blocks as occasion demands.
Accordingly, it is unnecessary to provide a plurality of windows which have heretofore been needed for partition, and it is possible to open only a desired block containing a constituent member necessary to repair or adjust in the space hermetically sealed by the hermetically sealing device to the atmosphere by remote control, for example. Thus, it is possible to minimize the waste of a replacement gas at the time of repairing or adjusting a constituent member and to rapidly replace the air by the replacement gas again.
More specifically, the partition device may comprise, for example, a partition wall which is approximately perpendicular to the optical path, and a closing member for hermetically closing an opening provided in the partition wall at a position coincident with the optical path as occasion demands. When measurement of the irradiation dose of illuminating light from the light source reveals that the output of the light source has become excessively large through some mistake, the optical path is shut off by closing the opening that is closest to the light source. By doing so, damage to an optical member can be prevented.
A second projection exposure apparatus of the present invention for attaining the second object of the present invention includes a light source (1) for emitting illuminating light including a wave band having absorbability with respect to oxygen (i.e., light IL having a center wavelength of 193.4 nm and a wave bandwidth of the order of from 193.0 nm to 193.8 nm, or light IL having a center wavelength of 193.4 nm and a wave bandwidth narrowed to several tens of pm), an illumination optical system (9a to 9c, 3, etc.) for applying the light from the light source to a mask (4) formed with a pattern, and a projection optical system (12) for forming a pattern image of the illuminated mask onto a substrate (5). The projection exposure apparatus further includes a gas supply system (10) for supplying an optical path extending over from the illumination optical system to the projection optical system with an inert gas having lower absorption characteristics than oxygen with respect to the wave band of the illuminating light, and a hermetically sealing device (18) which is disposed in the space between the substrate-side end of the projection optical system and the neighborhood of the substrate for replacing almost all atmosphere existing in the optical path of illuminating light in that space by a substance (an inert gas, e.g., nitrogen) other than oxygen.
According to a first preferred embodiment of the above-described second apparatus, the hermetically sealing device has a partition wall (19) for shutting off the space from the atmosphere, and a transparent member (16c) for transmitting illuminating light, and the gas supply system supplies the inert gas into a hermetic space which is formed by the partition wall and the transparent member.
According to a second preferred embodiment of the second apparatus, the apparatus further includes a focus detecting system (14 and 15) in which measuring light is incident obliquely on a substrate surface through the hermetic space formed by the hermetically sealing device, and the reflected light from the substrate is received through the hermetic space, thereby optically detecting the heightwise position of the substrate. The hermetically sealing device has a first light-transmitting portion (16a) for transmitting the measuring light entering it toward the substrate, and a second light-transmitting portion (16b) for transmitting the measuring light reflected from the substrate surface.
According to a third preferred embodiment of the second apparatus, the transparent member is a member for adjusting image formation characteristics of the projection optical system.
According to a fourth preferred embodiment of the second apparatus, the gas supply system supplies the inert gas into the hermetically sealing device and also adjusts the refractive index of the inert gas in the hermetically sealing device.
According to a fifth preferred embodiment of the second apparatus, the hermetically sealing device consists essentially of a transparent device (16c) which transmits the illuminating light.
According to the above-described second projection exposure apparatus of the present invention, it is possible to minimize the absorption of exposure light by oxygen and the generation of ozone even if far ultraviolet light (light including a wave band having absorbability with respect to oxygen)- is used as exposure light. Further, if light used for various kinds of measurement passes through the space between the projection optical system and the substrate or the neighborhood of the space, there is no error in measurement as has heretofore been caused by fluctuation in the atmosphere.
According to the above-described first preferred embodiment, the hermetically sealing device has a partition wall (19) for shutting off the space from the atmosphere, and a transparent member (16c) for transmitting illuminating light, and the gas supply system supplies an inert gas into the hermetic space formed by the partition wall and the transparent member. Therefore, the undesired absorption of light by oxygen is reduced.
According to the above-described second preferred embodiment, the hermetically sealing device has a first light-transmitting portion (16a) for transmitting measuring light entering the space toward the substrate, and a second light-transmitting portion (16b) for transmitting measuring light reflected from the substrate surface. Therefore, it is possible to use an optical, oblique incident focus detecting system despite the provision of the hermetically sealing device, and the focus detecting system is free from a measuring error due to fluctuation in the atmosphere.
According to the above-described third and fourth preferred embodiments, it is possible to solve the problem of the absorption of exposure light by oxygen and to adjust image formation characteristics of the projection optical system simultaneously.
According to the above-described fifth preferred embodiment, it is unnecessary to use an inert gas because the hermetically sealing device consists essentially of a transparent member (16c) which transmits illuminating light.
To achieve the third object of the present invention, an optical system for an exposure apparatus may have one of the following constructions.
According to a first preferred aspect of an optical system for achieving the third object, the optical system is applied to an exposure apparatus in which a plurality of optical parts (202, 203, 204, 205, 206, 210, 211 and 207-209) are contained in a frame 241, and the frame 241 is divided into a plurality of chambers 242a-242g, and the plurality of optical parts are housed in the different chambers of the frame 241, and gas replacing means 224, 248a-248g, L202a-L202g for replacing gases in the respective chambers 242a-242g are provided.
According to a second preferred aspect, the optical system is applied to an exposure apparatus in which a plurality of optical parts (202, 203, 204, 205, 206, 210, 211 and 207-209) are contained in a frame 241, and the frame 241 is provided with a plurality of chambers 242a-242g interconnected through connection passages L201b-L201g, and the plurality of optical parts are housed in the different chambers 242a-242g of the frame 241, and there are provided a gas supply means 224 for supplying inert gas to a first chamber 241a disposed at one end of the group of the interconnected chambers 242a-242g and a discharge passage L201h for discharging gas from a second chamber 242g disposed at the other end of the group of the interconnected chambers 242a-242g, and lids 244a-244g and valves 245a-245g, 246a-246g for blocking the communication between the adjacent chambers through the connection passages L201b-L201g when the lids 244a-244g are opened and for permitting the communication between the adjacent chambers through the connection passages when the lids 244a-244g are closed are provided in association with at least one of the chambers 242a-242g. 
According to a third preferred aspect, there are provided oxygen density sensors 247a-247g for detecting density of oxygen in chambers 242a-242g having lids 244a-244g; and discharge switching means 248a, L202a, 248b, L202b, 248c, L202c, 248d, L202d, 248e, L202e, 248f, L202f, 248g, L202g disposed between the chambers 242a-242f and connection passages L201b-L201g at the second chamber 242g side, respectively, and for permitting discharge of gas into the adjacent chambers through the connection passages L201b-L201g at the second chamber 242g side when the oxygen density detected by the oxygen density sensors 247a-247g is less than a predetermined value and for discharging the gas in the chambers 242a-242g out of the frame 241 when the oxygen density is greater than the predetermined value.
According to a first aspect of an exposure apparatus for achieving the third object of the present invention, there are provided a flow rate sensor 249 for detecting a flow rate of gas discharged from a discharge passage L201h, and a control device for controlling to turn OFF a light source of the exposure apparatus when the flow amount detected by the flow rate sensor 249 is less than a predetermined value.
According to a fourth aspect of the optical system for the exposure apparatus, the optical system is applied to an exposure apparatus in which a plurality of optical parts (206, 207-209, 210, 211) are contained in a frame 241, and the frame 241 is divided into a plurality of chambers 242e-242g, and the plurality of optical parts are housed in the different chambers 242e-242g of the frame 241, and gas supply means 224, L204 for supplying inert gas to the chambers 242e-242g are provided, and, further, there are provided lids 244e-244g, a discharge passage L203 for discharging the gases in the chambers 242e-242g, valves 245e-245g for blocking supply of inert gas from the gas supply means 224, L204 when the lids 244e-244g are opened and for permitting the supply of inert gas from the gas supply means 224, L204 when the lids 244a-244g are closed, oxygen density sensors 247e-247g for detecting oxygen density in the chambers 242e-242g, and discharge switching means 248e, L202e, 248f, L202f, 248g, L202g disposed between the chambers 242e-242g and the discharge passage L203 and for permitting discharge of gas from the discharge passage L203 when the oxygen density detected by the oxygen density sensors 247e-247g is less than a predetermined value and for discharging the gas from the frame 241 when the oxygen density is greater than the predetermined value.
According to a second aspect of the exposure apparatus, the exposure apparatus has the optical system according to any one of the above-mentioned first to fourth aspects.
Incidentally, while Figures illustrating various embodiments were used for facilitating the understanding of the present invention, the present invention is not limited to such embodiments.